In a medical scene, a doctor often compares computed tomography (CT) images captured at different times for determination of the course of a disease of a patient and so forth. At this time, if the compared region is a lung or the like of the patient, position variation due to heartbeat and breathing is included in the CT images. Thus, for example, image registration is carried out by transforming the image about a specified area in such a manner that the positions of feature points (e.g. blood vessels) in the respective compared CT images are made to correspond with each other. This allows the doctor to easily compare even the CT images in which the position variation is included regarding the specified area. The related art is disclosed in Japanese Laid-open Patent Publication No. 2013-141603